(also electrokinetic potential), the portion of the total potential drop at an interface between two phases that determines the relative displacement of the two phases in electrokinetic phenomena.

The total potential drop across a phase boundary in disperse systems results from the existence of an electrical double layer. The zeta potential is the potential drop across the diffuse portion of a double layer; within the diffuse portion, a liquid may be displaced tangentially relative to the phase boundary when external influences act on a system. The sign of the zeta potential may be reversed—that is, surface charge reversal may occur—under the influence of ions that are strongly adsorbed on a surface or when the pH of a liquid changes.

The zeta potential at an isoelectric point is equal to zero.